Examining the Sustainability of Copper Use for Disease Management and Horticultural Benefit in Tart Cherry Systems

Examining the Sustainability of Copper Use for Disease Management and Horticultural Benefit in Tart Cherry Systems

Summary

Foliar copper fungicide programs were evaluated again in 2008 and determined to provide excellent control of cherry leaf spot. These results were incorporated into outreach programs throughout the state for tart cherry growers. After further examining alfalfa plants in greenhouse and field trials, we found these plants were not efficient in hyperaccumulating copper from soils into leaf or root tissues. We next investigated the potential for using copper-accumulating soil bacteria to accumulate copper, and copper-resistant bacteria were isolated from organic and conventional orchard soils; these bacteria provided initial evidence for the capacity to accumulate copper in laboratory growth media.

Objectives/Performance Targets

The following objectives were addressed in this proposal
1. To incorporate foliar copper sprays into tart cherry management programs on commercial and organic farms in Michigan and to evaluate its efficacy on key diseases including CLS and to investigate copper’s potential to reduce soft fruit at harvest.
2. To assess the potential of copper-hyperaccumulating plants in removing copper from agricultural soils in greenhouse experiments and at on-farm sites.
3. To educate tart cherry growers about the utility of copper in their management programs and about phytoremediation.
4. To increase the adoption of these practices in the long-term in an effort to promote agricultural sustainability.

Accomplishments/Milestones

Objective 1: In 2007, copper was incorporated as a fungicide spray into six treatments in a trial conducted on Montmorency tart cherry trees. Sprays were applied to a block of 13-yr-old Montmorency tart cherry trees at the Northwest Michigan Horticultural Research Station near Traverse City, MI. This experiment was designed to test the efficacy of copper fungicides as a possible substitute for conventional fungicides as part of a reduced-risk, IPM program. Sprays were applied with a handgun to run-off (equivalent to 300 gal/A) at 300-350 psi. Treatments were arranged in a randomized complete block using four replications of single-tree plots. Spray dates and respective growth stages were as follows: 15 May (early petal fall); 23 May (shuck split); 1 Jun (first cover); 11 Jun (second cover); 21 Jun (third cover), and 2 Jul (fourth cover). The projected harvest date was 17 Jul. Cherry leaf spot infection periods and their level of severity were identified on: 9 May (low); 26 May (low); 2 Jun (high); 19 Jun (low); 20 Jun (low); 4 Jul (low); 9 Jul (high); 12 Jul (low); 27 Jul (moderate); 2 Aug (low); 20 Aug (low); 24 Aug (moderate); 29 Aug (low); 4 Sep (moderate); 7 Sep (low), and 10 Sep (high). The summer was hot and dry. In the month of Jun, there were only 6 days with precipitation that totaled 1.4 inches. From 1 Jun to 11 Aug, there were 68 days with temperatures greater than 70 degrees. Leaf spot infection and defoliation were evaluated on 14 Sep by examining all leaves on 20 shoots from each single-tree plot.

For the second year, cherry leaf spot developed unusually late resulting in an interval of 74 days between the final spray and disease assessment. With these conditions, fungicides with strong protectant activity were expected to be among the best. The best overall control of cherry leaf spot infection and defoliation were achieved with two early sprays of Bravo, a cover spray of Captan plus tebuconazole, two sprays of Cuprofix Disperss, followed by a final spray of tebuconazole plus Captan. In general, sprays containing Bravo or Cuprofix were very effective in controlling disease. Combining Captan with Gem did not improve disease control over Gem alone. The combination of Elite plus Captan was more effective than Elite alone. The low rate of DPX-LEM 17 was least effective of all the treatments. The addition of Elite to DPX-LEM 17 improved efficacy over DPX alone. On 14 Sep, 100% of the remaining leaves on unsprayed control trees were infected with cherry leaf spot and more than 81% of the leaves had fallen. Cherry trees that are completely defoliated in the early fall are more susceptible to cold injury and may be lost in the following season. We observed phytotoxicity in the form of leaf chlorosis where DPX-LEM 17 was sprayed alone and a slight amount of leaf bronzing in the treatment where two Bravo sprays were followed by one spray of tebuconazole plus Captan, two sprays of Cuprofix Disperss, and concluding with one spray of tebuconazole plus Captan.

Work left to do: The copper experiments are completed as we have now evaluated coppers for cherry leaf spot control over three growing seasons with excellent results. More knowledge is needed on the environmental parameters linking copper use to phytotoxicity in order to accurately predict when weather conditions are conducive for phytoxicity. This information is needed in order for growers to apply copper more often and with more comfort.

Objective 2: Alfalfa plants were tested under greenhouse and field conditions for their ability to accumulate copper from soils and to partition the copper into their above-ground (leaf and stem) tissue. Plants were grown in soils containing approximately 60-120 ppm copper in the greenhouse experiments and in natural orchard soils in field experiments.

In both sets of experiments, plants, roots, and soil were sampled 30-90 days after planting. In two separate greenhouse experiments, levels of copper in alfalfa roots and shoots increased only slightly after 90 days growth in soil amended with copper.

In the field experiment, alfalfa plants were planted into pots placed under the drip-line of tart cherry trees that received two applications of copper fungicides. The alfalfa plants grew considerably well under the shady orchard conditions, but there was no evidence of copper accumulation into the roots or shoots of the plants.

At this juncture of the project, our key cooperator Dr. Clayton Rugh, who is an expert in phytoremediation, was denied tenure and left Michigan State University. In addition, due to the lack of positive results with the alfalfa, we turned our attention to the possibility of using bacteria to bind copper ions in soil. The literature shows species of bacteria are known to be resistant to heavy metals and capable of binding these metal ions.

We initiated a study to isolate and identify copper-resistant bacteria from soil samples collected from six organic tart cherry orchards in northwest Michigan. Many copper-resistant bacteria turn blue when grown on copper-amended growth medium in petri dishes in the lab. Copper-amended medium is blue due to the color of copper solutions. When copper-resistant bacteria absorb copper from the medium, the bacteria become progressively more blue as the medium loses its blue color. We hypothesized that copper-resistant bacteria could be used to bind copper present at elevated levels in contaminated soils. While the copper would not be removed from soils using this strategy, the copper will be bound to the bacteria and thus unavailable for uptake by cherry roots. This then acts as to protect the cherry roots from copper exposure.

A total of 120 bacterial isolates were recovered from soil, and they were tested for their ability to grow on media with high levels of copper: 250, 500, 750, and 1000 ppm. Ten isolates grew most vigorously on media with 1000 ppm copper; these bacteria produced blue or green colonies on the high copper media (Figure 1), suggesting that these species were taking up copper from the media. In order to test these 10 bacteria in the soil, a spontaneous antibiotic (rifampicin) resistant strain was developed from each of the selected isolates in order for bacterial populations to be tracked in soil. Further studies will involve testing the persistence of these bacteria in soil with the best-performing strains chosen for field studies.

Objectives 3 and 4: In 2007, PI’s Sundin and Rothwell gave nine Extension presentations throughout Michigan to educate growers about the use of copper to control cherry leaf spot in tart cherry orchards. In addition, two newsletter articles were written in the MSU IPM Fruit CAT Alert, an online and written publication which, in an average week, reaches over 800 people.

Presentations:
1. Disease control update for 2007. Spring Tree Fruit meeting, Grand Rapids, MI, 4-19-07.
2. Disease control update for 2007. Spring Tree Fruit meeting, Belding, MI, 4-26-07.
3. RAMP report, plant pathology update. Great Lakes Fruit and Vegetable EXPO, Grand Rapids, MI, 12-05-07.
4. Fungicide Resistance Issues in Important Cherry Diseases, Northwest Orchard and Vineyard Show, Traverse City, MI, 1-16-08.
5. Cherry leaf spot: life after DMI’s. Northwest Michigan Orchard and Vineyard show, Traverse City, MI, 1-17-08.
6. RAMP Management Team Meeting, Northwest Michigan Horticultural Research Station, Traverse City, MI, 1-18-08.
7. Aspects and management of bactericide and fungicide resistance in tree fruit pathogens. Michigan State University Department of Horticulture, East Lansing, MI, 2-28-08.
8. Tree fruit disease update for 2008. West Central Spring Tree Fruit meeting, Hart, MI, 3-06-08.
9. Copper Use in Tart Cherry Systems. Benzie-Manistee Horticultural Society Annual Meeting, Benzonia, MI, 3-11-08.

Newsletter articles:

1. Sundin, G.W., and N.L. Rothwell. 2007. Fungicide cover spray considerations for cherry leaf spot control. MSU IPM Fruit CAT Alert, 5-15-07 issue.
2. Rothwell, N.L., and G.W. Sundin. 2007. Postharvest sprays for cherry leaf spot. MSU IPM Fruit CAT Alert, 7-24-07 issue.

Impacts and Contributions/Outcomes

More growers are showing interest in copper for CLS control, particularly as the number of currently available fungicides is decreasing and the potential for resistance is high with the limited chemistries left. However, 2007 was an extremely warm year in northwest Michigan, and many growers hesitated to use copper under high temperatures. Therefore, despite results that show copper provides excellent control of CLS, adoption has not been as widespread as anticipated due to potential leaf phytotoxicity. To address the adoption issue, we have been developing a model to provide guidelines to optimally time copper for adequate control and minimal phytotoxicity.

To determine temperature thresholds for safe copper application, we applied 1.2 lbs of actual copper and 6 lbs. hydrated lime (as a safener) per acre to trees at specific temperatures (70, 75, 80, 85, and 90 degrees F). We rate new growth for phytotoxic presence and severity after the first and the second copper/lime applications. We intended to associate given temperature thresholds with level of phytotoxic severity. In addition, we also rated for powdery mildew, another key disease in tart cherry. Preliminary results suggest that copper and lime applications do not cause phytotoxicity at any of these temperatures.

Collaborators: